There have been known optical transmitters using a light-emitting diode as a light source and a multimode optical-fiber as a transmission medium in domestic communications, in communications inside of motor vehicles and the like using LAN (Local Area Network). As prior arts associated with the present invention, the following are known.
(1) A photosemiconductor device characterized by comprising: a pair of conductive leads arranged facing each other; a metal container having a reflecting surface and disposed at a distal end of one of the conductive leads; a photosemiconductor element disposed on the reflecting surface of the metal container, one electrode of the photosemiconductor element being connected to one of the conductive leads; and a wire for connecting the other electrode of the photosemiconductor element to the other conductive lead (see, e.g., Japanese Unexamined Patent Publication No. Sho 58(1983)-56483).
(2) A light-emitting element for optical fiber coupling, which, via a transparent member, leads a light beam emitted from a light-emitting element pellet disposed on a light-emitting element supporter, the light-emitting element characterized by comprising the transparent member having a flat disc-shaped central portion and a peripheral portion, the flat disc-shaped central portion facing the light-emitting element pellet, the peripheral portion having a thickness decreasing as the distance from the center of the transparent member increases (see, e.g., Japanese Unexamined Patent Publication No. Sho 59(1984)-180515).
(3) A semiconductor light-emitting device characterized by comprising: a case; a light-emitting element accommodated in the case; lead members for supplying electric power to the light-emitting element from the outside; a connecting portion defined on the case for connecting an optical fiber to the case, the optical fiber guiding a light beam emitted from the light-emitting element so that the light beam is radiated via the optical fiber connected to the connecting portion; and a concave reflecting surface facing a light-emitting surface of the light-emitting element for reflecting the light beam emitted from the light-emitting element and thereby directing the light beam to a light-receiving surface of the optical fiber (see, e.g., Japanese Unexamined Patent Publication No. Hei 1(1989)-241185).
(4) A range finder measuring distances to a plurality of points, characterized by comprising a plurality of light-emitting sources and at least one photoreceptor, the light-emitting source being semiconductor chips respectively mounted on the bottoms of a plurality of recesses formed in a lead frame (see, e.g., Japanese Unexamined Patent Publication No. Hei 3(1991)-188312).
(5) A projector characterized by comprising (i) a half or less than a half of a paraboloid of revolution obtained by dividing a paraboloid of revolution by a face containing the axis of revolution, the half or less than half having a specular surface in the form of a paraboloid, and (ii) a light source arranged near a focal point on the paraboloid so that a center luminous flux of light emitted from the light source is incident on the specular surface (see, e.g., Japanese Unexamined Patent Publication No. Sho 62(1987)-17721).
(6) An optical coupling device for guiding, to a light-receiving surface at a proximal end of an optical fiber, a light beam emitted from a light-emitting element, the optical fiber being arranged facing the light-emitting element, the optical coupling device characterized by comprising: a transmissive-type light-collecting means arranged between the light-emitting element and the optical fiber for allowing a light beam emitted from the light-emitting element to pass through the transmissive-type light-collecting means so that the light beam is collected onto the optical fiber; and a reflective-type light-collecting means arranged around the transmissive-type light-collecting means, the reflective-type light-collecting means having a reflecting surface for reflecting a light beam radiated from the light-emitting element so that the light beam is collected onto the optical fiber, the reflecting surface of the reflective-type light-collecting means being a rotary elliptical surface, the rotary elliptical surface forming two focal points, the light-emitting element being arrange on one of the focal point, the light-receiving surface of the optical fiber being arranged on the other focal point (see, e.g., Japanese Unexamined Patent Publication No. 2002-40299).
(7) A structure for mounting a light-emitting diode comprising: a first lead frame having a chip-mounting seat with a photo penetration hole formed though the chip-mounting seat; an LED chip mounted on the first lead frame so that a light-emitting surface of the LED chip faces the photo penetration hole of the first lead frame; and a second lead frame bonded to a rear electrode of the LED chip by a wire, the structure characterized in that the LED chip, the wire, and distal ends of the respective first lead frame and second lead frame are covered with a transparent resin (see, e.g., Japanese Unexamined Patent Publication No. Sho 60(1985)-12782).
As a common optical transmitter for coupling an LED (light-emitting diode) and an optical fiber, there is known, for example, an optical transmitter, shown in FIG. 21, which is produced by transfer mold.
The optical transmitter 101 shown in FIG. 21 comprises a lead frame 105, an LED 103 arranged on the lead frame 105, a mold resin 109 that covers the lead frame 105 and the LED 103, a lens 104 formed of the same resin that forms the mold resin 109, and an optical fiber 102. Light beams radiated from the LED 103 are collected by means of the lens 104 onto the optical fiber 102.
However, there is a problem with this optical transmitter that it is difficult to couple light beams radiated form the LED 103 into the optical fiber 102 with high efficiency.
This problem is associated with a far field pattern (FFP) of an LED shown in FIG. 22. That is, the far field pattern of the LED is the Lambert Pattern where generally, the radiation intensity of the LED is represented by the cosine function. The far field pattern of the LED has a feature that a light beam from the LED has a wide radiation angle when compared with one from a semiconductor laser or the like. For this reason, in the optical system shown in FIG. 21, which is produced by transfer mold, a light beam having a wide radiation angle, of light beams emitted from the LED 103, cannot be coupled to the lens 104, giving rise to a loss.
One approach to this problem is to bring the lens 104 closer to the LED 103 for coupling a light beam having a wide radiation angle to the lens 104. However, bringing the lens 104 closer to the LED 103 causes the failure to ensure in a direction of the thickness of the optical transmitter a space required for wire-bonding of an electrode of the LED 103 to the lead frame. Further, the above approach requires that the lens 104 has a reduced focal length (i.e., that the lens 104 has a flattened curvature). With all these things considered, it is difficult to take the approach of bringing the lens 104 closer to the LED 104 to increase the efficiency of coupling in the optical fiber 102.
On the other hand, there have been proposed various methods of changing the optical path of a light beam having a wide radiation angle by reflecting the light beam from a concave mirror to increase the utility efficiency of light beams.
As a common optical transmitter utilizing a concave mirror, there is known, for example, one shown in FIG. 23.
In an optical transmitter 201, shown in FIG. 23, a substrate 205 has a concave portion in a part thereof. The concave portion serves as a concave mirror 108 having high reflectance and having an inside diameter gradually increasing from a bottom surface side thereof toward an upper edge side thereof. A LED 103 is disposed so that a rear surface side of the LED 103 (opposite a light-emitting surface 106) is in contact with the bottom surface side of the concave portion.
Of light beams radiated from the light-emitting surface 106 of the LED 103, a light beam having a wide radiation angle is reflected from the concave mirror 108 so that its optical path is changed toward a distal end of an optical fiber, not illustrated. Thus, even a light beam having a wide radiation angle can also be effectively used.
However, in an optical transmitter using a concave mirror, it is difficult to increase the coupling efficiency while achieving miniaturization and cost reduction of the optical transmitter.
That is, for example, in the optical transmitter 201 shown in FIG. 23, assuming that: the LED 103 is in the shape of a cube with a height of 300 μm and a width of 300 μm; the concave mirror 108 has a cone angle θ of 60°; and the concave portion has an inside diameter of φ 500 μm on the bottom surface side thereof, the concave portion needs to have a depth T0 of about 1.3 mm and an inside diameter R0 of 2 mm on the upper edge side in order for the concave mirror 108 to change the optical path of a light beam having a radiation angle of 45° or wider, of light beams radiated from the center of the light-emitting surface 106 of the LED 103. As a result, a lens with a diameter of φ 2 mm or greater is needed to collect and then couple into the optical fiber the light beams the optical paths of which have been changed by the concave mirror 108.
If the inside diameter of the concave mirror 108 on a side at which radiated light beams are raised, i.e., on the upper edge side, is as large as about φ 2 mm, there is a risk that, when a lens with a short focal length is used to collect light beams, the light beams may not be able to be coupled into the optical fiber because an NA incident to the optical fiber becomes so large that the light beams cannot be coupled into the optical fiber. On the other hand, when using a lens with a long focal length in order to take an advantage of a small incident NA, it is difficult to miniaturize the optical transmitter inclusive of the optical fiber.
Here, if the concave mirror 108 is curved, the optical transmitter can be slightly miniaturized, but is still larger than the one shown in FIG. 21. Further, a space needs to be provided at a portion of the concave mirror 108 for wire-bonding the electrode of the LED 103, which complicates the production process and widens a variation in transmission efficiency and thus a variation in the quantity of light coupled into the optical fiber. That is, depending on precisions in the location and shape of the concave mirror 108, the direction of light varies to result in variation in transmission efficiency, which widens a variation in the quantity of light coupled into the optical fiber. This creates the need to increase the dynamic range in optical transmission. Also, the substrate 205 needs to have therein the concave mirror 108, which results in a high cost. As has been explained above, in an optical transmitter using a concave mirror, after all, it is difficult to increase the coupling efficiency while achieving miniaturization and cost reduction of the optical transmitter.
Other than optical transmitters having the concave mirror, there are known one having a parabola-shaped mirror for improvement in coupling efficiency, one in which a mirror is provided on a side of a lens, and the like. These optical transmitters are problematic in that the number of components is increased to result in upsizing and a rise in the cost of the optical transmitters.
On the other hand, there is known an optical transmitter having (i) a lead frame with an opening formed therein and (ii) an LED having a light-emitting surface joined to the lead frame so that a light beam radiated from the light-emitting surface passes through the opening to be coupled into an optical fiber. In such an optical transmitter having the LED arranged on a rear surface of the lead frame, wire-bonding is made on the rear surface side opposite to the front surface side of the lead frame, i.e., to the side at a lens is provided. As a result, the lens (or the optical fiber) can be arranged close to the LED without considering a space required for the above-mentioned wire-bonding so that a relatively high coupling efficiency can be obtained.
However, only forming the opening through the lead frame for a light beam to pass through the opening does not make it possible to guide a light beam with a wide radiation angle to the lens at an effective angle. Therefore, it is eventually difficult to utilize a light beam with a wide radiation angle.
Meanwhile, in an optical transmitter, dissipation of heat generated in a light-emitting element is also important. If the optical transmitter is poor in heat dissipation, the temperature of a light-emitting element chip itself rises. Therefore, the magnitude of an electric current allowed to pass through the light-emitting element is limited, and the environment in which optical transmitters can be used is limited to make it impossible to use them in environments where the temperature is high such as motor vehicles and production facilities. For this reason, there is a need to reduce a thermal resistance of the LED chip and members around the LED chip. As a method of reducing the thermal resistance, there is known a method by disposing the LED chip on a substrate of a material good in heat dissipation. However, with this method there is a problem in that the thermal resistance of the LED chip itself cannot be reduced and that a high cost and upsized optical transmitters are resulted.
On the other hand, in the conventional optical transmitter 101 shown in FIG. 21, in which a surface of the LED 103 is covered with the mold resin 109, there is a problem in that a great thermal stress is created on the LED 103 when the ambient temperature varies, due to a large difference in a linear expansivity generally present between the LED 103 and the mold resin 109. For example, GaAs generally used in a red LED has a linear expansivity of about 6 ppm/K, while the material of the transparent mold resin such as an epoxy resin or the like has a linear expansivity of 60 ppm/K to 65 ppm/K, values substantially an order of magnitude greater. For this reason, there is a problem in that, in environments where wide temperature variations (e.g., from −40° C. to 110° C.) are encountered as expected in vehicle-installed devices and the like, a great thermal stress is applied onto the light-emitting surface of the LED 103 to make the light emission state unstable or make the LED 103 broken, or a bonding wire is ruptured due to a difference in linear expansivity between the bonding wire and the epoxy resin. Thus, it is difficult to gain a high reliability.